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The tasty crab that looks like an ugly frog

7 January 2015

IS IT a frog? Is it a crab? One look at a frog crab leaning forward on its front legs explains its name, but how these curious animals evolved has long been a mystery.

Ranina ranina, pictured, is the largest and best-known of the frog crabs. Its long, narrow body and paddle-like limbs look uncannily frog-like but its hard shell and 10 legs mark it out as a crab, and a true one at that, rather than a crab only in name like a hermit crab. Most true crabs have bodies at least as wide as they are long, and they scuttle sideways. Frog crabs, however, are slimmer and move forward and back.

People used to think that frog crabs were very primitive because of their odd-looking long bodies, says Javier Luque of the University of Alberta in Canada. Now, having reanalysed the fossil record, Luque has shown that they actually evolved after what we think of as modern crabs.

He says frog crabs evolved their narrow bodies as they adapted to burrowing into ocean-floor sediments, starting about 125 million years ago (Journal of Systematic Palaeontology, doi.org/xzz).

This article appeared in print under the headline “The crab that thinks it’s a frog”